In order to be able to ensure a flawless and safe operation in the electric power system of a motor vehicle, the determination of the state of charge, which is usually also referred to as the SOC (state of charge), of on-board electric power system batteries or traction batteries, in particular as a component of the detection of the battery status, plays an important role. In the automobile sector, there are already a number of known methods for accomplishing this determination. For a given initial condition of the state of charge and a known capacity of the battery, it is possible by means of integration of the current, for example, to determine the change of the state of charge under load during, for example, charging or discharging. In addition, there is the method of so-called 100 percent recalibration for a fully charged battery. Another common method is the measurement of the battery open-circuit voltage, which enables very accurate information about the current state of charge of a battery.
A drawback of these known methods is that they are either very inaccurate or can be carried out only under very special prerequisites. For example, the methods for determining the state of charge by integrating current lead to an error that increases over time and, in the medium term, results in a marked deviation from the actual state of charge of the battery. The known recalibration methods could thereby offer a remedy for this, but they are restricted to special environmental and system conditions, such as, for example, a fully charged battery or open-circuit voltage condition. However, in order for a recalibration by measuring the open-circuit voltage of the battery to be possible at all, the characteristic between the state of charge and the battery open-circuit voltage for the respectively employed cell chemistry has to permit a clear assignment between the state of charge and the battery open-circuit voltage. In the case of some cell chemistries, such as, for example, in the case of a lithium iron phosphate (LiFePO4) rechargeable battery, this is not the case owing to the flat characteristic curve.
In this connection, DE 10 2013 206 189 A1, for example, describes a method for determining a state of charge of a rechargeable battery, in which the state of charge is determined on the basis of a voltage differential, that is, on the basis of the derivative of the voltage curve of the rechargeable battery, by way of a suitable variable.
Moreover, DE 11 2013 006 736 T5 describes a method and a device for estimating a remaining battery capacity, in accordance with which the battery capacity is determined when switching between a charging and a discharging of the battery. In this case, the determination of the capacity is based on the detection of the discharge voltage of the battery. Here, too, the determination of the capacity can occur only in a specific situation, namely, when switching between charging and discharging of the battery.